Lids covering an integrated circuit die of an integrated circuit package are often used, among other functions, to encapsulate and protect the die from moisture and contaminants. Lids may also provide the additional function of a heat sink. Heat sinks are used in a variety of electronic devices to dissipate heat from the integrated circuit, preventing any failure of or damage to the integrated circuit. Heat sinks are made of a variety of heat conducting materials and are formed in a variety of shapes to adequately conduct the heat from the device. A lid is often used with an integrated circuit to reduce the heat of the semiconductor die to enable the integrated circuit to function properly. One purpose of the lid is to keep a semiconductor junction, such as a junction of the transistor, below a maximum specified operating temperature.
FIG. 1 is a top plan view of an integrated circuit package 100, shown as a flip chip package. Flip chip packages are thermally enhanced ball grid arrays (BGAs) with die facing down, and are provided with an exposed metal heat sink at the top. Because of their layout, flip chip packages lend themselves to the application of external heat sinks, either passive or active, for further heat removal efficiency. The integrated circuit package 100 comprises a region 102 having a plurality of contact pads 104 for receiving a flip chip die, and contact pads 108 for receiving other components, such as chip capacitors. As shown in the cross-sectional view of FIG. 2, a lid 201 is coupled to a substrate 202 and comprises an inner surface 203 and inner walls 204 forming a recess 206. The lid further comprises a foot portion 208 coupled to the substrate by a bonding agent 209, such as an adhesive. An integrated circuit die 210 has a plurality of contact pads 212 and corresponding solder bumps 214 which make contact with contact pads 216 of the substrate. An adhesive 218 may also be used above the integrated circuit die 210, and an underfill 220 may be used below the integrated circuit die. A solder mask 222 may also be used during the assembly process, as is well known in the art. As is also shown, the substrate is a multilayer substrate having conductors formed on a plurality of layers. A capacitor 224 on the surface of the substrate is coupled to the integrated circuit die by a conductor 226 on a layer of the substrate. The capacitor is also coupled to a ground plane 228 and a corresponding contact 230 on the bottom portion of the substrate. Other conductors 232 may extend directly from contact pads on the top of the substrate to contact pads 234 on the bottom of the substrate according to substrate artwork created for the substrate. The substrate artwork defines the various conductors so that the proper connections are made from the contact pads of the integrated circuit die (by way of the solder bumps and contact pads on the top of the substrate) to the contacts on the bottom of the substrate. Conductors 236 in various layers of the mulitlayer substrate, as well as a power contact 238, are also shown.
However, lids of integrated circuit packages often exhibit the problem of separating from the top of the package. Because the adhesion strength of the adhesive is sometimes not strong enough to secure the lid to the package, the conductive lid may come off of the package. Degradation of the adhesive may be caused by many factors, including improper dispensing of adhesive that does not produce full adhesive surface as designed, shear stresses introduced by thermal expansion/shrinkage differences at the interfaces, stresses introduced by warping, adhesive degradation due to chemical attack, etc. Excessive component handling stresses during test, assembly, or rework, such as excessive torque or pressure, may also result in separation of the lid from the package. In order to secure the lid to the substrate using an adhesive, significant area on the substrate is required for the adhesive to make a suitable bond. As more components such as chip capacitors continue to be added to substrate under the lid, the space for attaching the lid to the substrate using adhesives continues to decrease. Further, efforts are continuously being made to reduce the size of integrated circuit packages.
Accordingly, there is a need for an improved integrated circuit package and method of attaching a lid to an integrated circuit package.